How to organize the synchronization of files and folders with the possibility of permanent deletion of data?

Question

Data synchronization

I periodically make backups of files and folders on my home computer to an external hard drive, using the freefilesync program for this. Synchronization is carried out according to the "mirror" method, i.e. the backup on the hard disk is brought into full compliance with the contents on the main computer. However, as a result of synchronization, all files on the backup are deleted in the usual way, i. they can be restored using recovery utilities. The freefilesync program does not have the ability to organize the permanent deletion of files. Searching the Internet, I came across a person with the same problem, who suggested that the program developers implement the permanent deletion function, for which they recommended him to use data encryption. But I'm not interested in encryption. I couldn't find working solutions on google, or didn't search well.
1. A synchronization program that can permanently erase data. This has not been found yet.
2. A program that combines the ability to compare directories and permanently erase data. Since I use the "mirror" synchronization method, the algorithm seems to be as follows: first compare the directories, and based on the results of the comparison, display a list of files in the backup directory, which can then be permanently deleted. The conditions for filtering these files are: a) they do not exist on the primary computer; b) the modification date of these files differs from the modification date of the same files on the primary computer.
There are two more solutions that don't work for me.
1. Before backing up, first completely format the partition to which the backup is being made. The method is not suitable, since the amount of data is quite large, and it is time-consuming to make a full backup every time.
2. After backup, overwrite the free space on the partition with zeros, for example, with the ccleaner program. The method is not suitable for the same reason as indicated in the first paragraph, namely, the high duration of the erasing procedure, albeit shorter in time than in the first paragraph.
How to be?

Answer 1

The freefilesync program does not have the ability to organize the permanent deletion of files.

Well, you need to add it. The program is open source. There is an enumerated type in structures.h :

enum DeletionPolicy
{
    DELETE_PERMANENTLY,
    DELETE_TO_RECYCLER,
    DELETE_TO_VERSIONING
};

It is then processed in synchronization.cpp :

switch (deletionPolicy_)
        {
            case DELETE_PERMANENTLY:
                deleted = zen::removeFile(filepath); //[!] scope specifier resolves nameclash!
                break;

            case DELETE_TO_RECYCLER:
                deleted = recycler.recycleItem(filepath, relativePath); //throw FileError
                break;

            case DELETE_TO_VERSIONING:
                deleted = getOrCreateVersioner().revisionFile(filepath, relativePath, onNotifyFileCopy); //throw FileError
                break;
        }

Add one more enumerated value, type DELETE_SAFE , add processing and rebuild.